The recent growth in the demand for broadband services has resulted in a pressing need for increased capacity on existing communication channels. The increased bandwidth of fiber optic communication networks is still often insufficient to cope with this demand without utilizing the ability of these fibers to carry large numbers of individual communication channels, each identified by a discrete wavelength and each channel with a known bandwidth and separation from its adjacent channels. This technique is known as dense wavelength division multiplexing (DWDM) and standard channel designations for public networks are set by the International Telecommunications Union for channel spacings ranging between 200 GHz and 12.5 GHz.
The disadvantage associated with DWDM networks is that the increasing density of wavelength channels places increasing demand on network functionality for connecting the individual channels to individual destination points on a dynamic basis, and for the ability to add or drop an individual wavelength channel into or out of the optical signal. Currently these functions are primarily performed by electronic techniques but the demand for increased network speed and bandwidth calls for the development of techniques to perform these functions in the optical domain.
Two main techniques that have been developed to address this need are optical beam deflectors such as micro electromechanical (MEMS) mirror arrays or liquid crystal spatial light modulators (SLMs). Optical fibers deliver an input DWDM signal to a device, where the wavelength channels are demultiplexed with appropriate optics and directed to the beam deflector. The deflector routes the individual channels to a desired one many output ports. Since this routing is performed in free space it allows multiple signal beams to be simultaneously interconnected with minimal cross-talk between data channels.
As the demand for communications bandwidth increases, it will be required that more optical data channels are included on each optical fiber. This is likely to occur in stages and it will be common to see optical fibers carrying 32 individual wavelength channels in the same network as fibers carrying up to 128 wavelength channels or more. Standard liquid crystal devices are limited in the pixel density and fill factors that can be efficiently manufactured, thus severely limiting their functionality in DWDM applications. MEMS devices on the other hand can be manufactured with higher pixel densities. However, these devices are then limited to use in switching signals containing a predetermined number of channels i.e. a MEMS device that has been constructed for use in a system containing 64 individual wavelength channels cannot then be readily used where the signal contains 128 wavelength channels.
A relatively recent advancement in liquid crystal devices is the Liquid Crystal on Silicon (LCoS) SLM. The LCoS device is a reflective device where a liquid crystal is sandwiched between a transparent glass layer with a transparent electrode and a silicon substrate divided into a 2-dimensional array of individually addressed pixels. LCoS technology enables very high resolution devices with pixel pitch on the order of 10-20 μm and optical fill factors greater than 90%. Because of the high pixel densities possible with LCoS devices they have traditionally found application in high-definition display systems such as data and video projectors. The high pixel resolution and fill factors possible however, makes them uniquely suited to a channel by channel switching device which can be dynamically reconfigured to accommodate higher density DWDM applications.
A difficulty in transplanting this technology arises when the operating environments of LCoS devices in display applications is compared with that of telecommunications applications. In display applications, the LCoS is exposed to high intensities of light, including significant quantities of ultraviolet (UV) light. Thus, the typical lifetime of a display device only ranges between 5 to 10 years. Telecommunications components for deployment in a public communications network typically are rated for a useable lifetime of around 20 to 25 years. Also, they are generally only exposed to low intensities of light in the near infrared region. Thus, there are different failure mechanisms for the liquid crystal components between the technologies that must be accounted for and understood before deployment.
In display applications, such as high definition data and video projectors and televisions, calibration schemes for liquid crystal SLM components are focused on the brightness and contrast response of the SLM, and to check for pixel failure. They specifically concentrate on the phase response of the individual liquid crystal pixels to give a desired attenuation of incident light in order to display an image. The calibration is not typically concerned with corrective capabilities for the whole optical system in the device, nor is it usual to provide factors in the calibration for the long-term stability of the liquid crystal component to ensure that it remains relatively insensitive to phase variations or degradation over time.
The use of liquid crystal devices in telecommunications applications still requires a calibration of the phase response. This calibration, however, needs to be much more rigorous in order to accurately apply complex wavefront modification functions to the device. The functions may perform the multiple-port routing of wavelength channels, along with aberration correction functionality for the supporting optical system. Problems that can occur in the application of liquid crystal devices to DWDM switching applications can be attributed not only to irregularities in the liquid crystal device itself such as irregular phase response between pixels, but rather to aberrations from the surrounding optical elements as well. For example, surface irregularities of the optical elements or slight misalignments of the optical system are common causes of wavefront aberration. Since the system generally requires large beam sizes (>5 mm) to provide the required spectral resolution for a 50-GHz channel spacing, it displays significant sensitivity to these effects. Edge effects in the SLM pixels can also adversely affect the device performance. Consequently, it is necessary to have accurate knowledge of the individual pixel response across the whole face of the SLM.
Another of the challenges that needs to be overcome with liquid crystal technology is the temperature stability over a broad range of operating conditions. The phase response of the liquid crystal can vary significantly over typical operating temperatures for telecommunications devices which need to be accounted for and corrected.